Investigation of PM_2.5 and carbon dioxide levels in urban homes

Abstract

PM_2.5 (particulate matter with an aerodynamic diameter <2.5 μm) samples were collected in the indoor environments of 15 urban homes and their adjacent outdoor environments in Alexandria, Egypt, during the spring time. Indoor and outdoor carbon dioxide (CO₂) levels were also measured concurrently. The results showed that indoor and outdoor PM_2.5 concentrations in the 15 sites, with daily averages of 45.5 ± 11.1 and 47.3 ± 12.9 µg/m³, respectively, were significantly higher than the ambient 24-hr PM_2.5 standard of 35 µg/m³ recommended by the U.S. Environmental Protection Agency (EPA). The indoor PM_2.5 and CO₂ levels were correlated with the corresponding outdoor levels, demonstrating that outdoor convection and infiltration could lead to direct transportation indoors. Ventilation rates were also measured in the selected residences and ranged from 1.6 to 4.5 hr⁻¹ with median value of 3.3 hr⁻¹. The indoor/outdoor (I/O) ratios of the monitored homes varied from 0.73 to 1.65 with average value of 0.99 ± 0.26 for PM_2.5, whereas those for CO₂ ranged from 1.13 to 1.66 with average value of 1.41 ± 0.15. Indoor sources and personal activities, including smoking and cooking, were found to significantly influence indoor levels.

Implications: Few studies on indoor air quality were carried out in Egypt, and the scarce data resulted from such studies do not allow accurate assessment of the current situation to take necessary preventive actions. The current research investigates indoor levels of PM_2.5 and CO₂ in a number of homes located in the city of Alexandria as well as the potential contribution from both indoor and outdoor sources. The study draws attention of policymakers to the importance of the establishment of national indoor air quality standards to protect human health and control air pollution in different indoor environments.

Introduction

Indoor air quality (IAQ) has become a great concern in many countries. The governments of these countries recognized the health problems related to indoor air pollution and the importance to investigate and manage the IAQ of different indoor environments. Many recent studies were conducted worldwide regarding various aspects of indoor air quality and its effects upon health (Lee et al., 2002; Fromme et al., 2007; World Health Organization [WHO], 2010; Almeida et al., 2011; Franck et al., 2011; Canha et al., 2012). One important factor to maintain good IAQ is ventilation. Appropriate ventilation that adequately provides clean fresh air removes air pollutants present in indoor air, as well as reduces moisture and condensation, which in turn eliminates mould growth (Seppänen and Fisk, 2004). Building characteristics in terms of ventilation and air-conditioning systems can result in differences in ventilation levels between buildings and consequently lead to different IAQ. Indoor air pollutants can be produced by indoor sources or transferred from external emissions, as ventilation and infiltration are the main parameters for the exchange with outdoor air.

There are many sources of indoor air pollution, including various outdoor sources, smoking, cooking, heaters, building materials, furnishings, office equipment, and human activities. The sources vary depending on the type of building, season, and human style. Many studies also indicated that the indoor air quality of residential homes vary in relation to outdoor air quality (Wallace, 1996; Chao et al., 1998; Jones et al., 2000).

Carbon dioxide (CO₂) is an ubiquitous compound in air that is formed by combustion processes and human metabolism. Occupants are usually the main indoor source of CO₂, resulting in an increase of indoor CO₂ concentrations compared with outdoor levels. CO₂ can act as an indicator of ventilation efficiency, showing whether the supply of outside air is sufficient to dilute indoor air contaminants. Elevated CO₂ levels exceeding 1000 ppm can result from high occupancy combined with inadequate ventilation. Acceptable CO₂ concentrations in outdoor air typically range from 300 to 500 ppm. High CO₂ concentrations in the outdoor air can be an indicator of combustion and/or other sources of the contaminant.

Particulate matter (PM) is considered one of the main air pollutants that can exist in indoor environments in a way that can affect human health. The fine fraction of the outdoor and indoor PM, defined as PM with an aerodynamic diameter less than 2.5 µm (PM_2.5), is generated by various sources. Fuel combustion processes in energy production and transportation are the primary sources of the outdoor PM_2.5, whereas cooking and smoking activities contribute primarily to the indoor PM_2.5 (Afshari et al., 2005; Barraza et al., 2014). The fine PM fraction typically contains a mixture of particles emitted indoors as primary particles from different kinds of combustion-related activities (e.g., tobacco smoking and operation of gas stoves for cooking) and secondary particulates generated by chemical reactions of gas-phase precursors emitted both indoors and outdoors (e.g., sulfates and nitrates) (Weschler and Shields, 1997). Ambient (outdoor) fine particles can enter indoor environments through an open window or by the air-conditioning system as well as by infiltration through cracks and fissures in the building structure (Lai et al., 2012). The latter relates to how old and tight the building is; older buildings are very susceptible to direct penetration from outdoor fine PM. Outdoor PM_2.5 can largely contribute to indoor PM concentrations when air exchange rates are high (Abt et al., 2000; Meng et al., 2005). In fact, the increase of indoor fine PM concentration is mainly affected by outdoor traffic emissions, which contribute to a considerable amount of airborne fine particles in urban areas (Riley et al., 2002; Perez et al., 2010). When indoor sources are present, indoor PM concentrations can be higher than outdoor PM concentrations (Zhang et al., 2010; Abdel-Salam, 2012, 2013). Fine particles, in both indoor and outdoor air, were found to have the most serious effects on health, including increasing rates of cardiovascular and respiratory mortality and morbidity, as these are easily inhaled into the alveolar region (Heidi, 2000; Pope et al., 2002, 2009; WHO, 2006).

In Alexandria, industrial emissions and motor vehicles are important causes of ambient air pollution. High traffic density and high percentage of old cars and poorly maintained vehicles are primary causes of high outdoor levels of PM_2.5 and CO₂. Such emissions have significant implications for the indoor air quality, particularly in naturally ventilated buildings (Chang, 2002). Very few studies focused on indoor environments, which are expected to be the most important exposure sites for a substantial percentage of the population. Currently, limited data are available on the general understanding about IAQ in local residences, where most people including the susceptible group of elderly and children spend most of their time indoors. Investigation of IAQ and ventilation together with potential emission sources in homes has become an urgent matter. This can be of great help to suggest effective mitigation and control measures to improve IAQ and protect human health. The aim of this study is to investigate indoor and outdoor concentrations of PM_2.5 and CO₂ at a number of naturally ventilated household residences situated in a densely populated residential and commercial area. It also aimed to study the contribution of outdoor sources to indoor PM_2.5 and CO₂ levels as well as to determine potential indoor sources.

Experimental Methods

This study was conducted in Alexandria, the second largest Egyptian city, overlooking the coast of the Mediterranean Sea in the north-central part of the country, during the spring season from 25 March to 30 May 2010. Measurements of PM_2.5 and CO₂ were conducted in 15 homes located in one residential and commercial urban area known as Al-Ibrahimia (31°12′46″N, 29°55′42″E) in the eastern part of the city and about 1–2 km from the north coast. Sampling sites are flats situated in different floors of multistory residential buildings, some of which contain a number of commercial shops in the ground floors. For each site surveyed, various items of information about the sites were noted using a questionnaire. Questions included the ventilation type, volume of living rooms, number of smokers, cleaning, and other potential indoor pollution sources. The buildings are all supplied with natural gas as a source of fuel for cooking and other human activities. Floors of living rooms in all studied homes were covered with carpets. During the sampling period, all homes relied on natural ventilation through opening windows, which is very common in the domestic environment because of warm temperature during this period of the year. Brief information and main characteristics of the investigated flats are shown in Table 1.

Table 1. Main characteristics of homes used in this study

Site No.	No. of Occupants	No. of Children	No. of Smokers	Floor	Home Age (yr)	Volume of Living Room (m^3)	Frequency of Home Cleaning	Exhaust Fan/Hood in the Kitchen	Distance from Major Roads (m)	AER (hr^−1)
Home 1	3	0	0	5th	7	70	1 × week	Yes	60	3.1
Home 2	5	2	0	4th	5	65	1 × day	Yes	110	3.8
Home 3	6	3	2	7th	12	35	1 × week	No	240	2.2
Home 4	3	1	0	2nd	21	61	1 × week	Yes	40	3.3
Home 5	4	1	1	8th	9	50	1 × day	No	150	2.4
Home 6	2	0	0	5th	16	48	Infrequent	Yes	25	3.4
Home 7	4	0	1	3rd	4	45	1 × week	Yes	300	2.8
Home 8	5	1	1	6th	25	40	1 × day	No	120	2.4
Home 9	4	0	0	4th	15	55	1 × week	Yes	100	3.5
Home 10	4	1	1	4th	11	42	1 × week	No	150	2.9
Home 11	3	0	0	2nd	5	52	Infrequent	Yes	160	3.7
Home 12	5	2	2	6th	8	40	1 × week	No	220	1.6
Home 13	4	2	0	5th	15	47	1 × week	No	100	3.5
Home 14	2	0	0	8th	10	72	Infrequent	Yes	150	4.5
Home 15	3	0	1	7th	3	63	1 × day	Yes	50	4.0

A portable Q-Trak monitor (model 8551; TSI Inc., Shoreview MN, USA) was used to monitor the indoor/outdoor CO₂ concentrations at approximately 1 m vertically above the ground based on the mechanism of nondisperse infrared detection. Before sampling, the Q-Trak was calibrated with standard CO₂ gas at a known concentration. Pre and post zero checking of the CO₂ analyzer was also carried out. Moreover, simultaneous indoor and outdoor PM_2.5 samples were taken at each sampling site by using the Marple PM_2.5 environmental monitor samplers (model 200 personal environmental monitor [PEM]; MSP Co., Minneapolis, MN, USA) (Marple et al., 1987). As all households were occupied during the sampling process, only one pair of air samplers were used in this study to avoid any inconvenience to the occupants. Air was drawn through Teflon membrane filters (37 mm diameter and 2 µm pore size) at a flow rate of 10 L/min for 24 hr to collect airborne PM_2.5. Flow rates were monitored at the start and end of each sampling period with a calibrated rotameter. Each home was monitored on two to three occasions, and average values were then calculated. Indoor samplers were placed in the living room of each home at a height of approximately 1.5 m above the floor in order for the sampling to occur in the breathing zone of a seated person and to avoid potential interferences from excessive resuspension of particles. Similarly, outdoor measurements were conducted in the balconies, just outside the living rooms. All filters were conditioned in a room with controlled temperature and relative humidity for 24 hr before they were weighed. Each filter was weighed three times by a high-precision analytical balance (Mettler-Toledo AT261; Mettler-Toledo Inc., Columbus, OH, USA) prior to and after sampling, and average values were calculated. The readability of the balance was 0.01 mg. The gravimetric analysis included the weighing of unexposed blank filters to remove weighing errors produced by differences in temperature and humidity between weighings. Blank filters were obtained by sampling with power off and other conditions are similar to routine sampling. Filter handling was performed with great care to avoid damage, contamination, or the dislodging of collected particles before final weighing was completed. Also, air exchange rates were measured in living rooms of all selected homes at the beginning, middle, and end of the sampling period by the tracer gas decay method, and average values were then calculated. Sulfur hexafluoride (SF₆) was used as the tracer gas due to the fact that it does not exist naturally in the background air. One sampling point was chosen in the middle of the living room at a height of 1.5 m above the floor. The tracer gas was then dosed up to a level of about 10 ppm and allowed to decay, and the decay rate was used to calculate the air exchange rate. A photoacoustic multigas monitor (Innova 1412i; LumaSense Technologies Inc., Santa Clara, CA, USA) was used to measure the SF₆ level during the decay process.

Results and Discussion

Indoor/outdoor concentrations

Average mass concentrations of indoor and outdoor PM_2.5 of total samples in this study are shown in Figure 1. Error bars are also shown on the figure to show the variation in PM_2.5 concentrations for each selected home during the measurement period. Average indoor and outdoor PM_2.5 concentrations in the 15 sites were 45.5 ± 11.1 and 47.3 ± 12.9 µg/m³, respectively. Average indoor PM_2.5 levels ranged from 25 to 65 µg/m³ (median: 48 µg/m³), whereas those found outdoor ranged from 29 to 71 µg/m³ (median: 47 µg/m³). Currently there is no indoor or outdoor PM_2.5 standard in Egypt. However, the U.S. Environmental Protection Agency (EPA) National Ambient Air Quality Standards require the 24-hr PM_2.5 average concentration to be less than 35 µg/m³ to provide increased protection against health effects associated with long- and short-term exposures. Daily average indoor and outdoor PM_2.5 concentrations in the 15 flats were found to exceed the EPA standard by 80% and 73%, respectively. This may indicate the influence of heavy traffic and increased vehicle emissions, on both indoor and outdoor air, which might be expected in commercial areas. Similarly, Figure 2 shows daily averages of both indoor and outdoor CO₂ concentrations in the 15 homes. Error bars are also shown on the figure to show the variation in CO₂ concentrations for each selected home during the measurement period. Average indoor and outdoor CO₂ concentrations in the 15 residences were 583 ± 87 ppm (median: 560 ppm; range: 473–730 ppm) and 414 ± 47 ppm (median: 417 ppm; range: 330–490 ppm), respectively. A good indicator of proper ventilation is the level of CO₂ present, as elevated levels of CO₂ may indicate that additional ventilation is required. American National Standards Institute/American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ANSI/ASHRAE) Standard 62.1 (2013) recommends an indoor level of or less than 700 ppm above outdoor CO₂ concentration in the ambient air, which is typically about 300–500 ppm. This did not occur during the measurement period with no exceedances to the ASHRAE standard.

Figure 1. Daily average indoor and outdoor PM_2.5 concentrations in 15 homes.

Figure 2. Daily average indoor and outdoor CO₂ concentrations in 15 homes.

Impact of outdoor air

Indoor and outdoor sources are responsible for high PM_2.5 and CO₂ concentrations in indoor environments. The small size of the PM_2.5 fraction increases the potential to penetrate from outdoor to indoor environments (Cyrys et al., 2004; Hoek et al., 2008). This is facilitated by opening doors and windows during natural ventilation and presence of cracks and fissures in old buildings. The increased indoor concentrations of PM_2.5 and CO₂ in homes were probably related to outdoor air condition. The relationship between outdoor and indoor concentrations was investigated using linear regression analysis. Scatter plots between average indoor and outdoor concentrations for both PM_2.5 and CO₂ are shown in Figures 3 and 4, respectively. The regression lines are shown in the figures. Good correlation is expected to be found where frequent exchange of air occurs between the indoor and the outdoor environments. The relationship between the indoor and outdoor concentrations of PM_2.5 and CO₂ was indicated by the corresponding correlation coefficient (r) value. Significant positive Pearson bivariate correlations for PM_2.5 (r = 0.60, P = 0.05) and CO₂ (r = 0.71, P = 0.01) were found, which implied that outdoor levels of both PM_2.5 and CO₂ were in good correlation and influenced indoor levels, since the air in naturally ventilated households is replenished to varying degrees with ambient air without being filtered or conditioned.

Figure 3. Scatter plot between average indoor and outdoor PM_2.5 mass concentrations.

Figure 4. Scatter plot between average indoor and outdoor CO₂ concentrations.

The average indoor/outdoor (I/O) ratios of the observed sites were found to be 0.99 ± 0.26 (median: 0.87; range: 0.73–1.65) and 1.41 ± 0.15 (median: 1.37; range: 1.13–1.66) for PM_2.5 and CO₂, respectively. I/O ratio is an indicator for the strength of indoor sources, which could highly vary depending on the indoor source and outdoor concentration levels. I/O ratios above 1 were found for only six homes (homes 3, 5, 7, 8, 10, and 12) for PM_2.5 and in all homes for CO₂, indicating presence of potential indoor sources and human activities. I/O ratios below 1 were found for the other nine homes (homes 1, 2, 4, 6, 9, 11, 13, 14, and 15) for PM_2.5 only and varied from 0.73 to 0.94, indicating lower indoor PM_2.5 concentrations than outdoor ones and hence less influenced by indoor sources. Table 2 shows the I/O ratios of both PM_2.5 and CO₂ in all selected homes. Distance from major roads was found to correlate significantly with outdoor PM_2.5 concentrations (r = −0.75, P = 0.01), whereas this parameter was not significantly correlated with indoor PM_2.5 and CO₂ levels, possibly due to impacts from indoor sources. This was confirmed for homes with I/O <1, where significant correlation was found between indoor PM_2.5 concentrations and distance from major roads (r = −0.65, P = 0.05). The impact of site elevation on indoor concentrations of fine particulate matter and CO₂ was also investigated. The selected homes in this study were classified into seven groups (from 2 to 8) according to the floor level, and average concentrations at different floor levels were then calculated. No significant correlation was found between the outdoor concentrations and floor level. Also, no significant difference in the indoor concentrations at different elevations was observed. Similarly, the age of buildings in which selected homes exist was found to be not significantly correlated with indoor PM_2.5 and CO₂ concentrations. This suggests that indoor sources and ventilation are important factors in influencing indoor PM_2.5 and CO₂ concentrations in these households.

Table 2. Indoor/outdoor (I/O) ratios of both PM_2.5 and CO₂ at different monitoring sites

	I/O Ratio
Site No.	PM2.5	CO2
Home 1	0.87	1.34
Home 2	0.77	1.37
Home 3	1.65	1.66
Home 4	0.77	1.35
Home 5	1.19	1.33
Home 6	0.73	1.45
Home 7	1.14	1.26
Home 8	1.19	1.62
Home 9	0.77	1.27
Home 10	1.12	1.52
Home 11	0.94	1.64
Home 12	1.29	1.43
Home 13	0.85	1.34
Home 14	0.76	1.13
Home 15	0.87	1.42

In the current study, air exchange rate (AER) was used to assess the extent of entering outdoor air into the indoor environment. Ventilation times varied from one home to another during this period of the year. This affected AERs and I/O ratios, as ventilation has the potential to transfer PM_2.5 and CO₂ generated outdoors to indoor environments and also mitigate concentrations of PM_2.5 and CO₂ generated indoors. AERs for living rooms of all selected homes are shown in Table 1. The average AER in the studied households was 3.1 ± 0.8 hr⁻¹ (median: 3.3 hr⁻¹; range: 1.6–4.5 hr⁻¹). Homes with average I/O ratios above 1 for PM_2.5 had lower AERs (<3 hr⁻¹), wherease higher AERs (>3 hr⁻¹) corresponded to those with average I/O ratios below 1. The results indicate that under lower air exchange rate conditions, the indoor environments tend to accumulate indoor PM_2.5 at a higher level compared with the case when there is a higher air exchange. For CO₂, indoor activities were found to be responsible for I/O ratios above 1 in all studied homes. However, high AERs could provide adequate ventilation to dilute indoor CO₂ concentrations, which resulted in lower I/O ratios.

Impact of indoor air

The presence of indoor sources could contribute to elevated indoor levels of air pollutants. Combustion sources including smoking and cooking generate and considerably increase the indoor PM_2.5 and CO₂ concentrations. Seven homes with smoking activity were found in the current study, with an average indoor PM_2.5 concentration of 50.0 ± 11.2 µg/m³ (median: 53 µg/m³; range: 33–65 µg/m³) and an average indoor CO₂ concentration of 653.3 ± 71.8 ppm (median: 680 ppm; range: 530–730 ppm). In homes where people smoke, including homes 3, 5, 7, 8, 10, 12, and 15, calculated I/O ratios of PM_2.5 and CO₂ were all above 1 except for home 15 with I/O ratio below 1 only for PM_2.5, as shown in Table 2. This is probably due to the high AER of 4 hr⁻¹ in home 15 combined with a relatively large volume of the living room. The I/O ratio in homes with smoking activity varied from 0.87 in home 15 to 1.65 in home 3 with an average value of 1.21 ± 0.23 for PM_2.5, whereas it varied from 1.26 in home 7 to 1.66 in home 3 with an average value of 1.46 ± 0.15 for CO₂. Significant correlation was found between smoking and average indoor concentrations of both PM_2.5 (r = 0.60, P = 0.05) and CO₂ (r = 0.84, P = 0.01). The other eight nonsmoking homes were found to have an average indoor PM_2.5 concentration of 41.5 ± 10.1 µg/m³ (median: 42 µg/m³; range: 25–52 µg/m³) and an average indoor CO₂ concentration of 521.0 ± 38.8 ppm (median: 515 ppm; range: 473–585 ppm). It was found that the average indoor PM_2.5 and CO₂ concentrations in smoking homes were higher than the average indoor levels of PM_2.5 and CO₂ in nonsmoking homes by 21% and 25%, respectively. This confirms the significant association and contribution of smoking to indoor PM_2.5 and CO₂ concentrations. Cooking using gas stoves is an essential indoor activity that was undertaken by occupants from two to three times per day in all selected homes. Insufficient ventilation in domestic kitchens can cause elevated levels of PM_2.5 and CO₂ in living rooms. This usually occurs if kitchens are not adequately ventilated through opening windows or by using exhaust fans or hoods to properly remove cooking fumes and combustion emissions. Significant differences were found in this study by applying the t test between PM_2.5 and CO₂ values in homes supplied with exhaust fans or hoods in kitchens (n = 9) and the corresponding PM_2.5 and CO₂ values in homes with no fans or hoods installed in the kitchens (n = 6). Significantly higher PM_2.5 levels (P = 0.019) were observed, with an average value of 53.3 ± 8.9 µg/m³ (median: 54.5 µg/m³; range: 38–65 µg/m³), in homes without exhaust fans/hoods in the kitchens than in those with installed fans or hoods, which have an average value of 40.2 ± 9.5 µg/m³ (median: 40 µg/m³; range: 25–52 µg/m³). Similarly, significantly higher CO₂ levels (P = 0.011) were observed, with an average value of 655.0 ± 79.1 ppm (median: 689.5 ppm; range: 530–730 ppm), in homes without exhaust fans/hoods in the kitchens than in those with installed fans or hoods, which have an average value of 534.6 ± 53.8 ppm (median: 520 ppm; range: 473–638 ppm). Therefore, properly installed fans or hoods in kitchens can largely reduce cooking emissions if they work efficiently through regular cleaning and routine maintenance.

As people tend to spend more time indoors, the density of human occupancy as well as number of children could also affect indoor levels of PM_2.5 and CO₂ in homes under investigation. Significant correlation was found between number of occupants and indoor CO₂ concentrations (r = 0.74, P = 0.01), as well as between number of children and indoor CO₂ levels (r = 0.58, P = 0.05). As humans exhale CO₂ as a bioeffluent gas, occupants are usually the main indoor source of CO₂, resulting in an increase of indoor CO₂ concentrations compared with outdoor levels. No correlation was found between number of occupants or children and indoor PM_2.5 concentrations. This is probably due to the regular cleaning in highly occupied homes, which reduced deposited particles on the surfaces of floors and furniture. This could greatly reduce the amount of resuspended PM_2.5 that can result from children and occupant-related activities. Variable volume of living rooms is another important factor that could influence levels of indoor pollutants. A significant negative correlation was observed between volume of living rooms and indoor concentrations of both PM_2.5 (r = −0.68, P = 0.01) and CO₂ (r = −0.72, P = 0.01). Undoubtedly, high human occupancy combined with small living rooms could considerably increase the indoor CO₂ concentrations, as shown in homes 3, 8, 10, and 12. Human activities such as smoking and cooking could also lead to high indoor levels of PM_2.5 and CO₂ in living rooms of small sizes. As both PM_2.5 and CO₂ are known to be generated from combustion sources, an association between indoor PM_2.5 and CO₂ levels could be found. Figure 5 shows a scatter diagram between average indoor PM_2.5 and CO₂ concentrations in the studied residences. A proper linear relationship was found, which implied a significant correlation between indoor PM_2.5 and CO₂ concentrations (r = 0.65, P = 0.01). Therefore, any preventive action applied to control emissions from indoor combustion activities could considerably reduce both indoor PM_2.5 and CO₂ levels.

Figure 5. Scatter plot between average indoor PM_2.5 and CO₂ concentrations.

Conclusions

Indoor air quality monitoring data currently available in Egypt are very limited and not adequate to permit policymakers to develop an accurate profile of the actual situation or to identify the major contributors to indoor air quality problems. The current study presents indoor and outdoor PM_2.5 and CO₂ concentrations in 15 naturally ventilated urban residences located in one residential and commercial area in Alexandria City during the spring season. Daily average indoor and outdoor PM_2.5 concentrations in the sites under investigation were found to be significantly higher than the EPA 24-hr ambient air quality standard for PM_2.5. Therefore, more efforts should be taken for the control of fine particle pollution, and more efforts should also be taken towards the establishment of indoor and ambient PM_2.5 concentration standards. Good correlation was found between indoor PM_2.5 and CO₂ concentrations and their corresponding concentrations outdoors. Furthermore, indoor PM_2.5 levels were found to be in a good association with indoor CO₂ levels, indicating their generation from similar indoor combustion-related activities. Therefore, any control measures taken to reduce internal combustion emissions could mitigate indoor levels of PM_2.5 and CO₂ simultaneously. Indoor activities were found in this study to contribute strongly to indoor levels of PM_2.5 and CO₂ in many instances. Calculated I/O ratios confirmed the significant contribution from indoor sources to both PM_2.5 and CO₂. A certain pattern was apparent of the indoor CO₂ levels being higher than those outdoors (I/O ratios >1) in all homes, and to a lesser extent for PM_2.5 with I/O ratios above 1 in only six homes. Smoking was found to be a strong contributor to elevated levels of indoor PM_2.5 and CO₂. Cooking using gas stoves is another important indoor combustion source of both PM_2.5 and CO₂ unless adequate ventilation by opening windows or efficient and well-maintained exhaust fans/hoods are supplied in the kitchens. Frequent cleaning is recommended in order to regularly remove deposited particles on surfaces and floors and reduce the potential of resuspension. Ventilation rate was also found to influence I/O ratios of PM_2.5 and CO₂, as high ventilation rates could dilute indoor levels. Further research is needed to confirm these findings and identify additional determinants of PM_2.5 and CO₂ concentrations, and evaluate how these findings can be translated into preventive actions. As there are no air quality standards currently exist in Egypt for indoor environments, the establishment of national IAQ standards is highly recommended to protect human health and control air pollution in different indoor environments.

Additional information

Notes on contributors

Mahmoud M. M. Abdel-Salam

Mahmoud M. M. Abdel-Salam is working in the Department of Environmental Sciences, Faculty of Science, Alexandria University, Egypt. He is interested in studying outdoor and indoor air quality.